Cellular networks are expanding as more users turn to mobile devices as their primary and secondary communications devices, thus requiring an increase in the amount of wireless infrastructure. The increased load demands on the infrastructure often causes network congestion. This mandates an increase to the capacity of the communications networks, such as the GSM/GPRS (General Packet Radio Service), to reduce interference. Different proposals to increase capacity and reduce interference include systems for frequency planning or quality of signal control, power control schemes, and mobile station performance enhancement systems.
Wireless and cellular networks also suffer from increased interference emanating from other users, such as neighboring cells on the same frequency, known as co-channel interference (CCI), or interference emanating from neighboring frequencies on the same cell, known as adjacent channel interference (ACI). A single-antenna interference cancellation (SAIC) system is sometimes used to counter CCI. The use of this type of system has been standardized by the 3G-partnership project (3GPP), as the technical specification group (TSG) for the GSM/EDGE radio-access network (GERAN).
SAIC can reduce interference using Downlink Advanced Receiver Performance (DARP) techniques, as adopted by the 3GPP. The 3GPP technical specification 05.05V8.11, entitled the Third Generation Partnership Project, Technical Specification Group GSM/EDGE Radio Access Network, Radio Transmission and Reception, is hereby incorporated by reference in its entirety. This specification describes the requirements for different components used in such communications systems, for example, the transceivers, base stations, and other components.
Different SAIC algorithms used as interference reduction systems have been proposed, including a constant modulus method and joint demodulation method (JDM), which uses the joint demodulation of the user and an interference sequence. These systems typically begin with a standard least-squares (LS) estimate of a propagation channel, and a static channel profile for an interferer. A joint branch metric can be minimized using modified Viterbi systems, and any estimated sequences for an interfering signal in a least mean squares (LMS) algorithm can update channel estimates for a desired interfering propagation channel.
Such systems are disclosed in U.S. Pat. No. 7,006,811, U.S. Patent Publication No. 2004/0170234, and an article by the Nokia Research Center entitled Co-Channel Interference Suppression For Constant Modulus Signals. These systems describe an iterative method for improving channel impulse response (CIR) estimation. In such systems, the timing offset of a training sequence is presumed to be perfectly known. This assumption, however, cannot always be held, and a valid assumption could be that the timing offset is randomly distributed over a number of symbols. Thus, the CIR and timing offset could be jointly estimated.